Here I shall be controversial (for a Western fan) and throw my vote behind First Gundam. That was a genuinely groundbreaking series, but Zeta (which has its moments) turned Gundam into a franchise. From that point onward, just about every Gundam TV project has been severely compromised in one way or another.
Some Gundam series managed to be reasonably interesting despite this, like the delightfully demented G Gundam, thanks to having rebellious creative teams unafraid of pissing off anyone from fans to Bandai. Other shows, most notably 00, are absolute shells of what the same creative team could've produced outside the constraints of the Gundam franchise.
Actually, I hear that a lot from Western fans. (...The ones that don't think SEED is the greatest series ever, that is.)
My favorite? 08th MS Team. That's the one that doesn't at some point mess it all up. Mobile Suit Gundam was groundbreaking...but I'm a youngster (comparatively). I know what it did, but that's just facts in my head--it doesn't resonate.
Zeta Gundam's a great series, but it bends over backwards to get dark on me. The ending irritated me, but more importantly I REALLY HATED how in Zeta Gundam the Argama crew treated Kamille like crap from start to finish, despite him being an ace pilot.
Gundam Wing has a really special place in my heart, but looking back a lot of it was kinda like...psuedo-intellectual, maybe? (Possibly the wrong word.) Like it worked hard to be deep but the substance wasn't there. I love it, but denying that would be silly.
G Gundam? My thoughts on that one are comedic. Likely best not said here--people love this series a lot.
ZZ Gundam I've not finished, but while I love it thus far, its clearly not on the level of MSG and Z, even though I far prefer this cast to either series.
Never watched Victory, Turn A or X.
Gundam 00 feels like Wing done right--with more of a focus on the characters and their relationships with fewer gaping plotholes. But I hear they botched it at the finish line with S2. Someone would have to tell me if that's right or not.